Peel-off packaging system for fresh foodstuffs

ABSTRACT

A sealing product to be sealed on at least one ring-shaped sealing zone which surrounds the opening of a container, made from a sealing sheet including a support layer of a fibrous material of mainly vegetable fibers and a multi-layer structure including, on one of its faces, an adhesive layer and, on its other face, a sealing layer, whereby the sealing product is stamped to make a relief whose form is such that its base follows, at least discontinuously, a contour close to that of the internal edge of the ring-shaped sealing zone, the structure of the sealing sheet including additionally an intermediate layer made of plastic having a good strength, able to undergo plastic deformation, and of which the average thickness is such as it endows the entire sealing product with sufficient malleability and resistance for it to be possible to stamp it to the desired shape without tearing.

The invention relates to the field of lids designed to seal containers. It concerns more particularly lids which take the form of peel-off lids designed to seal containers, typically pots or bottles, which contain fluid products such as fresh dairy products or other food products with a limited shelf life, requiring neither pasteurization nor sterilization.

Generally, foodstuffs with a limited shelf life like non-sterilized or non-pasteurized dairy products, for example fresh milk products, are poured into containers (pots, or bottles) typically made of glass or plastic. These containers are then covered with a lid which is made either from a metalloplastic sheet including a layer of aluminum alloy, or from a composite sheet comprising paper and plastic (typically PET: polyethylene terephthalate), and which is heat-sealed onto a zone of the container which surrounds the upper edge of the cavity of the container, that we will thereafter refer to as the “ring-shape sealing zone”. These lids adhere to the container with enough force to ensure leak tightness, in particular against steam, but sufficiently weakly to allow them to be separated from said ring-shaped sealing zone by peeling. As this invention lends itself particularly well to containers provided with a collar (or neck) surrounding a dispensing opening, we will also indicate said ring-shaped sealing zone by the term “mouth”, which usually indicates the upper edge of the neck which surrounds said dispensing opening.

Typical rates of filling for such pots or containers for non-pasteurized or non-sterilized dairy products and other foodstuffs with a limited shelf life are typically about 30 containers per minute. Moreover, said containers can be filled and sealed simultaneously, so that the same machine, able to function for example with 24 tracks, is typically able to supply 40,000 pots per hour. It is clear that, for such rates, the conditions of positioning and heat sealing of the lid on the container are of paramount influence on the quality of the leak-tightness of the containers sealed in this way. The sealing product used may be either a sealing sheet which is placed on several containers at the same time before carrying out the simultaneous heat sealing of said containers, or a customized lid that is placed on the opening of one container alone.

Patent application WO2006/016038 describes the structure of a sealing product, whether a customized lid or a sealing sheet, which comprises:

-   -   a support structure, which may include a layer of aluminum         alloy, or a layer of paper, or a plastic layer having sufficient         mechanical characteristics, typically PET (polyethylene         terephthalate);     -   a multi-layer structure, typically co-extruded, including, on         one of its faces, an adhesive layer including an adhesive         material chosen so as to adhere to said support layer, and, on         its other face, a sealing layer including a sealing material         chosen so as to adhere, after heat sealing, to the ring-shaped         sealing zone.

In particular for customized lids, which are generally precut in the form of discs whose diameter is greater than that of the dispensing opening, the problem arises of placing said lids above the ring-shaped sealing zone with a high degree of accuracy. A badly positioned lid is likely, after heat sealing, not to adhere perfectly to the entire circumference of the ring-shaped sealing zone, so that the container cannot be sure to provide proper conservation of the product which it contains. It is all the more important to solve the problem in that production rates are high, that the positioning of the lid and controlling said positioning must be made in a fraction of a second and that, by the time that it is realized that a slight positioning defect has occurred, a very large number of rejects may have resulted.

The solution proposed up until now involved precutting the lid to a diameter significantly higher than the external diameter of the container mouth and folding back the “overlapping” part along the cylindrical wall outside the collar or neck. Only a metalloplastic structure could be appropriate for such a solution. Its aluminum support layer, of a minimum thickness of 30 micrometers, made it possible to permanently fold back said overlapping edge by crimping around the neck of the container, without damaging the sealed zone. On the other hand, a non-metalloplastic structure had the disadvantage of not being able to be crimped onto the neck, so that it was necessary either to accept unattractive overlapping lids, unreliable because they were likely to be inopportunely torn off during handling of the containers, or to precut lids of smaller diameter and to thus run the risk of having many badly sealed containers

But the metalloplastic structure used to solve this problem had disadvantages:

-   -   it is not easy to recycle;     -   as it contains a large amount of aluminum, its cost is         relatively high, and its carbon footprint is high so that it is         not in line with current concerns relating to sustainable         development;     -   it has a metallic appearance, difficult or expensive to modify         (lacquering, printing, etc.);     -   it has a low crack strength which may be observed in particular         during the transportation of sealed containers: because of the         vibrations occurring during transport, the aluminum alloy layer         may be subjected to strong forces and is therefore likely to         present cracking which no longer guarantees the desired sealing.

The purpose of the invention is to procure sealing products, in the form of sealing sheets or precut lids, which, while not having the above-mentioned disadvantages of the lids which have a metallo-plastic structure, allow sealing at high rates which is satisfactory from the standpoint of both the leak-tightness of the products sealed and that of appearance and reliability. Another object of the invention is to provide lids that are economical to manufacture and that comply with current directives for packaging materials concerning their respect for the environment.

According to the invention, the sealing product, designed to be sealed on at least one ring-shaped sealing zone which surrounds the opening of a container, is made from a sealing sheet comprising:

-   a) a support layer made of a fibrous material mainly including     vegetable fibers, typically a paper; -   b) a multi-layer structure, typically co-extruded, including, on one     of its faces, an adhesive layer including an adhesive material     chosen so as to adhere to said support layer, and, on its other     face, a sealing layer including a sealing material chosen so as to     adhere, after heat sealing, to the ring-shaped sealing zone.     characterized in that: -   c) said sealing product is stamped so as to produce a relief whose     form is such that its base follows, at least discontinuously, a     contour close to that of the internal edge of said ring-shaped     sealing zone; -   d) the structure of the sealing sheet additionally includes an     intermediate layer made of plastic having good strength and able to     undergo plastic deformation, of which the average thickness, which     is typically higher than a fifth of the average thickness of said     support layer when said average thicknesses are expressed in terms     of surface density, is such that it endows the entire sealing     product with sufficient malleability and resistance for it to be     possible to stamp it to the desired shape without tearing

The sealing product according to the invention is either a customized stamped lid, or a sheet or rather a plaque including several stamped cavities. It is made from a sealing sheet of which the structure comprises several layers of plastic and at least one support layer of paper or, more generally, a material mainly comprising vegetable fibers.

The material of the support layer is inexpensive and withstands better than a plastic the thermal shock caused by the heat sealing tool, with which it is directly in contact. The thermal shock is all the greater as this material, a poor heat conductor, requires the heat sealing tool to be brought up to a temperature higher than that used for heat sealing lids with a metalloplastic structure used in prior art. This is what is meant by “mainly includes vegetable fibers”: it may contain other fibers, synthetic organic fibers for example, but it must be able to withstand the temperatures and the pressure imposed by the heat sealing tool during heat sealing, which can exceed 210-230° C. Typically, it may be considered that this material includes at least 80% of vegetable fibers.

But such a material is poorly suited to stamping because it tears very easily. It is very important to be able to center the lid in relation to the ring-shaped sealing zone and this problem can easily be solved, without calling upon the use of sophisticated display and positioning tools, by creating a relief whose base is, at least intermittently, close to the interior of the cavity designed to contain the food product, which cavity is itself bordered by said ring-shaped sealing zone, so that the relief must have a base whose contour is close to that of the internal edge of said ring-shaped sealing zone. It can be deduced from the latter for example by designing a “parallel” contour, also called offset contour. It may also, while being inside the contour of the internal edge of the ring-shaped sealing zone, include only some suitably placed zones in the vicinity of said contour. Said zones are suitably placed in the meaning that, once the lid is placed on the upper edge of the container with said relief inserted into the container cavity and the base thereof being in contact—at least at some points—with the internal wall of the container at the level of its open end, the said lid cannot easily move in relation to said open end of the container.

Naturally, to limit the risks of tearing, the fibrous material is advantageously selected from materials comprising long fibers, such as paper commonly called “kraft paper”, and having good elongation at rupture characteristics for a paper, typically greater than 5%, whatever the direction of the test (length direction, width direction). This requires the use of a nonstandard paper because, in general, and as the example below shows, the elongation at rupture in the length direction—also called the machine direction—of a standard paper is lower. Additionally, the base of the stamped cavity is advantageously toroid in shape with a fillet radius typically greater than 5 times the total thickness of the sealing sheet. Also, advantageously, the tilted wall of the relief does not have too abrupt a slope in relation to the plans of the sealing sheet. Typically, one should aim for a tilted wall of less than 45° in relation to said plane.

Said support layer may have an average thickness which, expressed in terms of mass per surface unit, i.e. in terms of “surface density”, also called “basic weight” or “grammage”, lies between 40 and 110 g/m². Preferably, a layer having a thickness close to 70 g/m² will be chosen.

The sealing product according to the invention includes a multi-layer structure, typically co-extruded, with an adhesive material chosen so as to adhere, by one of its faces, to said support layer, and a sealing material chosen so as to adhere, by its other face, to the ring-shaped sealing zone during said heat sealing. This multi-layer structure also comprises, preferably adjacent to the adhesive layer made up of a binder having an affinity for the support layer, an intermediate layer made of plastic with a high strength and able to undergo plastic deformation, the thickness of which is such that it endows the entire sealing product with sufficient malleability and resistance for it to be possible to stamp it to the desired form without tearing. This intermediate layer makes it possible to peel off the lid without defibering or tearing. Typically, expressed in terms of surface density, the average thickness of said intermediate layer is greater than a tenth of the average thickness of the support layer, and preferably greater than a fifth of the average thickness of the support layer. Preferably, the strength is greater than 60 MPa, and preferably still greater than 80 MPa. The high malleability of the material advantageously results in an elongation at rupture greater than 5%, and preferably greater than 10%.

Preferably, said material belongs to the group of polyamides and thermoplastic polyesters. Of the polyamides, polyamide 6 turns out to be particularly suitable because it endows the sealing sheet with very good tearing strength, even at low thicknesses. Of the thermoplastic polyesters (saturated polyesters), a PET (polyethylene terephthalate) should preferably be chosen, cooled quickly (in order to give an amorphous structure or one with a low rate of crystallinity) and not bi-axially oriented.

The layer of adhesive contains a binder including at least one of the following compounds: grafted copolymers and/or copolymers and/or terpolymers including acid groups or ionomers, maleic-anhydride grafted PE metallocene, maleic-anhydride grafted linear PE, EVA copolymers, EMA copolymers, EEA copolymers, EBA copolymers. Said support layer may have an average thickness which, expressed in terms of surface density, lies between 3 and 15 g/m² and preferably in the region of 5 g/m².

Said sealing layer may, depending on the material of the container onto which said lid must be sealed, contain at least one of the following compounds: ethylene vinyl acetate copolymer (EVA), ethylene-methyl acrylate copolymer (EMA), ethylene-butyl acrylate copolymer (EBA), ethylene-ethyl acrylate copolymer (EEA), the functionalized terpolymers including acid groups or esters, acid copolymers of type EAA (ethylene-acrylic acid), EMA (ethylene-methyl acrylate), terpolymers including maleic anhydride. Said sealing layer may include at least one of said compounds and, optionally, a tackifying resin. Said sealing layer may have an average thickness which, translated into terms of surface density, lies between 3 and 20 g/m² and preferably in the region of 10 g/m². Said sealing layer may also include a mineral or organic load, for example talc, with a content by weight ranging typically from 5% to 30%, so as to encourage the cohesive rupture of said sealing layer when opening said packaging.

Said intermediate layer may have an average thickness ranging between 3 and 20 g/m², preferably in the vicinity of 15 g/m² for a support layer with an average thickness close to 70 g/m². Said multi-layer structure, typically obtained by co-extrusion, may have an average thickness ranging between 12 and 65 g/m², preferably in the vicinity of 35 g/m² for a support layer with an average thickness close to 70 g/m².

The container may be made of glass, metal or plastic, single or multi-layer or spiral. The plastic(s) is (are) chosen from saturated polystyrenes (SP), polyesters (for example PET), polyolefins, in particular polyethylene (PE) or polypropylene (PP), and polyvinyl chlorides (PVC).

FIGS. 1 to 3 schematically illustrate an example of an embodiment of the invention.

FIG. 1 represents the structure of the sealing sheet from which the lid of FIG. 2 was cut out and stamped.

FIG. 2 a is a vertical cross section of a lid according to the invention. FIG. 2 b is a top view of the same lid. FIGS. 2 a and 2 b illustrate an example of a lid with specific dimensions, which should not be taken as limiting the scope of the patent.

FIG. 3 shows how the lid of FIG. 2 fits onto the open end of a cylindrical jar.

EXAMPLE OF EMBODIMENTS

Lids (1) were made by cutting from a sealing sheet, the structure of which is shown schematically in FIG. 1. The lids were stamped in order to obtain the shape represented in FIG. 2 a.

The sealing sheet has a structure (10) which includes a support layer (11) made of paper and, adjacent to this, a multi-layer structure (16) made of plastic.

The paper used to make the support layer (11) is a paper with long fibers with a weight per square meter, i.e. a surface density, of 70 g/m². This paper has large quantities of long fibers which endow it with improved mechanical characteristics as compared to a standard paper of the same surface density. In the table below, it can be seen that the paper chosen within the context of the invention has mechanical properties that are less anisotropic than standard paper of the same weight per square meter, a much better tearing strength, and in particular a particularly good elongation at rupture in the length direction (MD), as compared to standard paper, and it is this last property which appears as best characterizing the quality of the special paper which is required to make up the support layer of the sealing product according to the invention:

Paper used within the context of the Standard paper invention Force at rupture (kN/m) Force at rupture (kN/m) MD 6.4 MD 5.6 CD 3.4 CD 4.0 Elongation at rupture (%) Elongation at rupture (%) MD 1.8 MD 5.5 CD 8.4 CD 9.0 Tearing strength (mN) Tearing strength (mN) MD 525 MD 950 CD 490 CD 1050 MD (machine direction) = length direction; CD = (cross direction) width direction

The multi-layer structure (16) comprises:

-   -   a layer (12) of binder, made of functionalized terpolymer         containing acid groups and esters, binding the support layer and         the intermediate layer, of average thickness 5 g/m²;     -   an intermediate layer (13) of polyamide 6 (PA6), of average         thickness 15 g/m²;     -   a layer (14) of binder, made of a material identical to that of         layer 12, binding the intermediate layer and the sealing layer;         of average thickness 5 g/m²;     -   a sealing layer (15) made of modified PE, including mineral         loads, of average thickness 10 g/m².

Thanks to its stamped shape, as illustrated in FIG. 2 a, the lid (20) can be easily centered in relation to the ring-shaped sealing zone (4) of the container (1) which it is designed to cover, without it being necessary to call upon sophisticated display and positioning tools. FIG. 3 shows how the lid (20) fits onto the open end (2) of a cylindrical jar (1): the base (21) of the stamped relief (24) follows—here over the entire circumference—a contour close to that of the inner edge (41) of the ring-shaped sealing zone (4), which corresponds here to the mouth (5). The contour of the inner edge (41) corresponds to the inside (3) of the cavity (6) designed to contain the foodstuff. The peripheral edge (22) of the lid, is the not stamped part of the lid which remains in the plane of the sealing sheet. It is placed above the mouth (5) then held up against this during heat sealing by means of the heat sealing tool (100).

For an external diameter (not including the tab) of around 34 mm, the stamped central part (27) has a depth (h) of approximately 2 mm in relation to the peripheral edge (22). The base (21) of the stamped cavity (25), a transition zone between the cavity (25) and said peripheral edge (22), advantageously toroid in shape with a fillet radius (R) of 0.5 mm. The tilted wall (23) of the cavity (25) is tilted at an angle α, here around 50°, in relation to said plane of the sealing sheet. The transition zone between the tilted wall (23) and the stamped central part (27) is also toroid in shape with a fillet radius (R′) close to R.

The lid (20) was cut out from the sealing sheet according to a shape such that it also has a gripping tab (26), making it possible to grip the tab to detach the lid by peeling off.

Lids of identical general shape but not stamped were also manufactured and heat-sealed onto the same containers, made by molding in high density polyethylene (HD PE). These lids had the following structures:

A) a metalloplastic structure: aluminum layer (30 g/m2)/co-extruded multi-layer (30 g/m2) B) a paper/co-extruded multi-layer structure, in which

-   -   the paper is the standard paper whose mechanical properties were         presented above     -   the extruded multi-layer has the following structure:         -   a. a layer of binder, made of functionalized terpolymer             containing acid groups and esters, binding the support layer             and the intermediate layer, of average thickness 5 g/m²;         -   b. an intermediate layer (13) of low density polyethylene,             of average thickness 15 g/m²;         -   c. a layer of binder, made from a material identical to that             of layer a) average thickness 5 g/m²         -   d. a sealing layer made of modified PE, including mineral             loads, of average thickness 10 g/m².             Comparative peelability tests were carried out and led to             the following observations:             Type A lids: Easy peeling, cohesive opening without tearing             of material;             Type B lids: Peeling impossible because the material tears;             Lids of the example according to the invention: Easy             peeling, cohesive opening without tearing of material.

It is observed that the material of the intermediate layer, a PA6 in this case, endows the paper/plastic complex according to the invention with tear-proof qualities as good as the metalloplastic structure, without the disadvantage of being expensive and difficult to recycle. Additionally, a metalloplastic structure (of type A) could also be stamped but this would require a thicker layer of aluminum (at least 40 μm) and would consequently increase the disadvantages referred to above.

In addition, lids of identical general shape were produced by cutting out from several sealing sheets (I, J, K, L, M) whose globally identical structures differ only by the thickness of the intermediate layer (13) made of polyamide 6:

a paper support layer (11) identical to that described previously: paper with long fibers, of weight per square meter 70 g/m², with the slightly anisotropic mechanical characteristics indicated above:

.1 force at rupture MD: 5.6 kN/m; CD: 4.0 kN/m; .2 elongation at break MD: 5.5%; CD: 9.0% a layer (12) of binder, made of functionalized terpolymer containing acid groups and esters, binding the support layer and the intermediate layer, of average thickness 5 g/m²; an intermediate layer (13) made of polyamide 6 (PA6), of average thickness 5 g/m² (structure I); 7 g/m² (structure J); 10 g/m² (structure K); 15 g/m² (structure L); 20 g/m² (structure M) respectively; a layer (14) of binder, made of a material identical to that of layer 12, binding the intermediate layer and the sealing layer; of average thickness 5 g/m²; a sealing layer (15) made of modified PE, including mineral loads, of average thickness 10 g/m².

After cutting, 20 lids from each structure were stamped with the same stamping tool, giving the form described above. After stamping, the following observation was made:

Structure I J K L M Thickness of 5 g/m² 7 g/m² 10 g/m² 15 g/m² 20 g/m² intermediate layer (13): Ratio 7% 10% 14% 21% 29% intermediate layer (13) thickness/ support layer (11) thickness Number of lids with 19 2 1 0 0 folds and/or tears

It can be noted that structure I does not make it possible to obtain lids without folds and tears after stamping. The results are clearly better as of structure J, i.e. as soon as the thickness of the intermediate layer PA 6 reaches one-tenth of the average thickness of the paper support layer. The results are excellent with structures that have an intermediate layer of thickness greater than, or equal to 15 g/m². 

1. Sealing product (20), designed to be sealed on at least one ring-shaped sealing zone (4) which surrounds the opening (2) of a container (1), made from a sealing sheet comprising: a) a support layer (11); b) a typically co-extruded multi-layer structure (16), comprising, i. on one of its faces, an adhesive layer (12) including an adhesive material chosen so as to adhere to said support layer, ii. an intermediate layer (13) made of plastic, and, iii. on its other face, a sealing layer (15) including a sealing material chosen so as to adhere, after heat sealing, to the ring-shaped sealing zone; characterized in that: c) said support layer is a fibrous material mainly comprising vegetable fibers; d) said intermediate layer (13) is made of a plastic with good strength, able to undergo plastic deformation, the average thickness of which is such that it endows the entire sealing product with sufficient malleability and resistance for it to be possible to stamp it without tearing so as to make a relief (24) whose form is such that its base (21) follows, at least discontinuously, a contour close to that of the internal edge (41) of said ring-shaped sealing zone.
 2. Sealing product according to claim 1, characterized in that said intermediate layer is made of plastic having a strength greater than 60 MPa, preferably greater than 80 MPa.
 3. Sealing product according to claim 1, characterized in that said intermediate layer is made of plastic having an elongation at rupture greater than 5%, preferably greater than 10%.
 4. Sealing product according to claim 1, characterized in that said intermediate layer is made of a plastic belonging to the group containing polyamides and thermoplastic polyesters.
 5. Sealing product according to claim 4 in which said intermediate layer is made of polyamide 6 (PA6).
 6. Sealing product according to claim 4 in which said intermediate layer is of polyethylene terephthalate (PET) not bi-axially oriented.
 7. Sealing product according to claim 6 in which said polyethylene terephthalate (PET) has been suddenly cooled in order to give an amorphous or slightly crystallized structure.
 8. Sealing product according to claim 1, characterized in that the average thickness of said intermediate layer is greater than a tenth, and preferably greater than a fifth of the average thickness of said support layer when said average thicknesses are expressed in terms of surface density.
 9. Sealing product according to claim 1, in which said support layer has an elongation at rupture of at least 5%, whatever the test direction.
 10. Sealing product according to claim 1, in which said support layer (11) has an average thickness which, expressed in terms of surface density, ranges between 40 and 10 g/m², and preferably close to 70 g/m².
 11. Sealing product according to claim 1, in which said intermediate layer (13) has an average thickness ranging between 3 and 20 g/m², and preferably in the vicinity of 15 g/m² for a support layer having an average thickness close to 70 g/m².
 12. Sealing product according to claim 1, in which said sealing layer (15) has an average thickness which, expressed in terms of surface density, ranges between 3 and 20 g/m², and preferably in the vicinity of 10 g/m².
 13. Sealing product according to claim 1, in which the base (21) of the relief (24) is toroid in shape with a fillet radius typically greater than 5 times the total thickness of the sealing sheet.
 14. Sealing product according to claim 1, in which the tilted wall (23) of relief (24) is tilted by less than 45° in relation to the plane of the sealing sheet.
 15. Sealing product according to claim 1, characterized in that it also comprises a gripping tab (26). 